The present invention relates to the field of turbine engines, and more particularly to variable vane stators for use therein.
In a fan or compressor component of a turbofan or turbojet engine it has been believed to be advantageous to angularly vary some stator vanes to increase the airflow capacity and pressure rise capability of the following rotor blades. This is usually done to improve the matching of vane stages at part-speed conditions in order to increase stall margin. A problem can arise for a conventional variable stator when this is the first such variable vane stage, as its inlet air angle remains nearly constant as the vane's stagger angle is varied, and thus the vane's incidence angle changes with its stagger. If this required stagger adjustment is substantial, the change in incidence will cause the vane to become stalled or choked, which can result in high aerodynamic losses in either case, and possibly may make the stagger adjustment less effective by reducing the turning capability of the vane.
Tameo in U.S. Pat. No. 4,652,208, teaches two axially adjacent rows of variable stator vanes for use in a gas turbine engine, to provide increased airflow. However, the provision of two rows of variable vanes rather than one increases the complexity of the engine, particularly since each row requires variable vane positioning control devices such as those described in the Tameo patent. Thus the suggested use of a fixed row of vanes (see Col. 1 of Tameo) would be more desirable from a design standpoint. However, if a row of variable vanes of Tameo were positioned adjacent a row of fixed inlet vanes, and if the angular positions of the variable vanes were varied substantially, the widths of the air passage slots between the variable vanes of Tameo would vary widely, to produce poor slot geometry, with accompanying aerodynamic losses.